A person's spine is not straight and its shape is slightly arched and concave. The arch or indentation at the bottom of the back changes from one person to the other. There are those with a small indentation at their lower back and others with a larger indentation. The pressure on a person's back when their sitting posture is wrong is very great and may cause irreversible damage. Therefore using a pillow at the arch of the lower back according to the desired size gives comfort and is healthy for the back and prevents the creation of pressure at the lower back arch. The back is constructed of a chain of vertebrae and by aligning the arch of the chair (small or large) with the height of the arch it is possible to reach a maximum adjustment of the back's arch with the support's arch. Using the chair according to the invention is particularly beneficial to people sitting for hours on end and prevents the creation of pressure on the lower back's vertebrae.